1. Field of the Invention
The present invention relates to a dielectric ceramic composition having a resistance to reduction and to a multi-layer ceramic capacitor or other electronic device using the same.
2. Description of the Related Art
A multi-layer ceramic capacitor, one type of electronic device, is being broadly used as a compact, large capacity, high reliability electronic device. The number used in each piece of electronic equipment has also become larger. In recent years, along with the increasing miniaturization and improved performance of equipment, there have been increasingly stronger demands for further reductions in size, increases in capacity, reductions in price, and improvements in reliability in multi-layer ceramic capacitors.
Multi-layer ceramic capacitors are normally produced by stacking a paste for formation of the internal electrode layers and a paste for formation of the dielectric layers using the sheet method or printing method etc. and then cofiring the internal electrode layers and dielectric layers in the stack.
As the electroconductive material for the internal electrode layers, generally Pd or a Pd alloy is being used, but Pd is high in price and therefore relatively inexpensive Ni, Ni alloys, and other base metals have been coming into use. When using a base metal as the electroconductive material of the internal electrode layers, sintering in the atmosphere ends up causing the internal electrode layers to oxidize and therefore the cofiring of the dielectric layers and internal electrode layers has to be done in a reducing atmosphere. If cofiring in a reducing atmosphere, however, the dielectric layers end up being reduced and becoming lower in specific resistance. Therefore, nonreducing type dielectric materials are being developed.
Multi-layer ceramic capacitors using nonreducing dielectric materials, however, suffer from a remarkable deterioration in the IR (insulation resistance) due to application of an electric field, that is, there are the problems that the IR lifetime is short and the reliability is low.
Further, if a dielectric is exposed to a direct current electric field, the problem arises of the specific dielectric constant .di-elect cons..sub.r falling along with time. If the dielectric layers are made thinner so as to reduce the size of the chip capacitor and increase its capacity, the electric field acting on the dielectric layers will become stronger when the direct current voltage is applied, so the change in the specific dielectric constant .di-elect cons..sub.r along with time, that is, the change in the capacity along with time, will end up becoming much larger.
Further, a capacitor is also required to be excellent in temperature characteristic. In particular, in some applications, it is desired that the temperature characteristic be smooth under harsh conditions. In recent years, multi-layer ceramic capacitors have come into use for various types of electronic equipment such as the engine electronic control units (ECU) mounted in the engine compartments of automobiles, crank angle sensors, antilock brake system (ABS) modules, etc. These electronic equipment are used for stabilizing engine control, drive control, and brake control, and therefore are required to have excellent circuit temperature stability.
The environment in which these electronic equipment are used is envisioned to be one in which the temperature falls to as low as -20.degree. C. or so in the winter in cold areas or the temperature rises to as high as +130.degree. C. or so in the summer right after engine startup. Recently, there has been a trend toward reduction of the number of wire harnesses used for connecting electronic apparatuses and the equipment they control. Electronic apparatuses are also being mounted outside of the vehicles in some cases. Therefore, the environment is becoming increasingly severe for electronic apparatuses. Accordingly, the capacitors used for these electronic apparatuses have to have smooth temperature characteristics in a broad temperature range.
As tempetature-compensating capacitor materials superior in temperature characteristics, (Sr, Ca)(Ti, Zr)O.sub.3 based, Ca(Ti, Zr)O.sub.3 based, Nd.sub.2 O.sub.3 --2TiO.sub.2 based, La.sub.2 O.sub.3 --2TiO.sub.2 based, and other materials are generally known, but these compositions have extremely low specific dielectric constants (generally less than 100), so it is substantially impossible to produce a capacitor having a large capacity.
As a dielectric ceramic composition having a high dielectric constant and a smooth capacity-temperature characteristic, a composition comprised of BaTiO.sub.3 as a main component plus Nb.sub.2 O.sub.5 --Co.sub.3 O.sub.4, MgO--Y, rare earth elements (Dy, Ho, etc.), Bi.sub.2 O.sub.3 --TiO.sub.2, etc. is known. Looking at the temperature characteristic of a dielectric ceramic composition comprising BaTiO.sub.3 as a main component, since the Curie temperature of pure BaTiO.sub.3 is close to about 130.degree. C., it is extremely difficult to satisfy the R characteristic of the capacity-temperature characteristic (.DELTA.C=.+-.15% or less) in the region higher in temperature than that. Therefore, a BaTiO.sub.3 based high dielectric constant material can only satisfy the X7R characteristic of the EIA standard (-55 to 125.degree. C., .DELTA.C=.+-.15% or less). If only satisfying the X7R characteristic, the material is not good enough for an electronic apparatus of an automobile which is used in the above-mentioned harsh environments. An electronic apparatus requires a dielectric ceramic composition satisfying the X8R characteristic of the EIA standard (-55 to 150.degree. C., .DELTA.C=.+-.15% or less).
To satisfy the X8R characteristic in a dielectric ceramic composition comprised of BaTiO.sub.3 as a main component, it has been proposed to shift the Curie temperature to the high temperature side by replacing the Ba in the BaTiO.sub.3 with Bi, Pb, etc. (Japanese Unexamined Patent Publication (Kokai) Nos. 10-25157 and 9-40465). Further, it has also been proposed to satisfy the X8R characteristic by selecting a BaTiO.sub.3 +CaZrO.sub.3 +ZnO+Nb.sub.2 O.sub.5 based composition (Japanese Unexamined Patent Publication (Kokai) No. 4-295048, No. 4-292458, No. 4-292459, No. 5-109319, and No. 6-243721). In each of these compositions as well, however, since the easily vaporized and diffusing Pb, Bi, and Zn are used, sintering in air or another oxidizing atmosphere becomes a prerequisite. Therefore, there are the problems that it is not possible to use an inexpensive base metal such as Ni for the internal electrodes of the capacitor and it is necessary to use Pd, Au, Ag, or another high priced precious metal.